RFC 7897

Internet Engineering Task Force (IETF) D. Dhody
Request for Comments: 7897 U. Palle
Category: Experimental Huawei Technologies
ISSN: 2070-1721 R. Casellas
CTTC
June 2016 Domain Subobjects
for the Path Computation Element Communication Protocol (PCEP)
Abstract
The ability to compute shortest constrained Traffic Engineering Label
Switched Paths (TE LSPs) in Multiprotocol Label Switching (MPLS) and
Generalized MPLS (GMPLS) networks across multiple domains has been
identified as a key requirement. In this context, a domain is a
collection of network elements within a common sphere of address
management or path computational responsibility such as an Interior
Gateway Protocol (IGP) area or an Autonomous System (AS). This
document specifies a representation and encoding of a domain
sequence, which is defined as an ordered sequence of domains
traversed to reach the destination domain to be used by Path
Computation Elements (PCEs) to compute inter-domain constrained
shortest paths across a predetermined sequence of domains. This
document also defines new subobjects to be used to encode domain
identifiers.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7897.

Further, the domain identifier may simply act as a delimiter to
specify where the domain boundary starts and ends in some cases.
This is a companion document to Resource Reservation Protocol -
Traffic Engineering (RSVP-TE) extensions for the domain identifiers
[RFC7898].
1.1. Scope
The procedures described in this document are experimental. The
experiment is intended to enable research for the usage of the domain
sequence at the PCEs for inter-domain paths. For this purpose, this
document specifies new domain subobjects as well as how they
incorporate with existing subobjects to represent a domain sequence.
The experiment will end two years after the RFC is published. At
that point, the RFC authors will attempt to determine how widely this
has been implemented and deployed.
This document does not change the procedures for handling existing
subobjects in the PCE Communication Protocol (PCEP).
The new subobjects introduced by this document will not be understood
by legacy implementations. If a legacy implementation receives one
of the subobjects that it does not understand in a PCEP object, the
legacy implementation will behave according to the rules for a
malformed object as per [RFC5440]. Therefore, it is assumed that
this experiment will be conducted only when both the PCE and the Path
Computation Client (PCC) form part of the experiment. It is possible
that a PCC or PCE can operate with peers, some of which form part of
the experiment and some that do not. In this case, since no
capabilities exchange is used to identify which nodes can use these
extensions, manual configuration should be used to determine which
peerings form part of the experiment.
When the results of implementation and deployment are available, this
document will be updated and refined, and then it could be moved from
Experimental to Standards Track.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2. Terminology
The following terminology is used in this document.
ABR: Area Border Router. Routers used to connect two IGP areas
(Open Shortest Path First (OSPF) or Intermediate System to
Intermediate System (IS-IS).
AS: Autonomous System
ASBR: Autonomous System Border Router
BN: Boundary node; can be an ABR or ASBR.
BRPC: Backward-Recursive PCE-Based Computation
Domain: As per [RFC4655], any collection of network elements within
a common sphere of address management or path computational
responsibility. Examples of domains include IGP area and AS.
Domain Sequence: An ordered sequence of domains traversed to reach
the destination domain.
ERO: Explicit Route Object
H-PCE: Hierarchical PCE
IGP: Interior Gateway Protocol. Either of the two routing
protocols: OSPF or IS-IS.
IRO: Include Route Object
IS-IS: Intermediate System to Intermediate System
OSPF: Open Shortest Path First
PCC: Path Computation Client. Any client application requesting a
path computation to be performed by a Path Computation Element.
PCE: Path Computation Element. An entity (component, application,
or network node) that is capable of computing a network path or
route based on a network graph and applying computational
constraints.
P2MP: Point-to-Multipoint
P2P: Point-to-Point

RSVP: Resource Reservation Protocol
TE LSP: Traffic Engineering Label Switched Path
XRO: Exclude Route Object
3. Detail Description
3.1. Domains
[RFC4726] and [RFC4655] define a domain as a separate administrative
or geographic environment within the network. A domain could be
further defined as a zone of routing or computational ability. Under
these definitions, a domain might be categorized as an AS or an IGP
area. Each AS can be made of several IGP areas. In order to encode
a domain sequence, it is required to uniquely identify a domain in
the domain sequence. A domain can be uniquely identified by an
area-id, AS number, or both.
3.2. Domain Sequence
A domain sequence is an ordered sequence of domains traversed to
reach the destination domain.
A domain sequence can be applied as a constraint and carried in a
path computation request to a PCE(s). A domain sequence can also be
the result of a path computation. For example, in the case of H-PCE
[RFC6805], a parent PCE could send the domain sequence as a result in
a path computation reply.
In a P2P path, the domains listed appear in the order that they are
crossed. In a P2MP path, the domain tree is represented as a list of
domain sequences.
A domain sequence enables a PCE to select the next domain and the PCE
serving that domain to forward the path computation request based on
the domain information.
A domain sequence can include boundary nodes (ABR or ASBR) or border
links (inter-AS links) to be traversed as an additional constraint.

Thus, a domain sequence can be made up of one or more of the
following:
o AS Number
o Area ID
o Boundary Node ID
o Inter-AS Link Address
These are encoded in the new subobjects defined in this document as
well as in the existing subobjects that represent a domain sequence.
Consequently, a domain sequence can be used by:
1. a PCE in order to discover or select the next PCE in a
collaborative path computation, such as in BRPC [RFC5441];
2. the parent PCE to return the domain sequence when unknown; this
can then be an input to the BRPC procedure [RFC6805];
3. a PCC or a PCE to constrain the domains used in inter-domain path
computation, explicitly specifying which domains to be expanded
or excluded; and
4. a PCE in the per-domain path computation model [RFC5152] to
identify the next domain.
3.3. Domain Sequence Representation
A domain sequence appears in PCEP messages, notably in:
o Include Route Object (IRO): As per [RFC5440], IRO can be used to
specify a set of network elements to be traversed to reach the
destination, which includes subobjects used to specify the domain
sequence.
o Exclude Route Object (XRO): As per [RFC5521], XRO can be used to
specify certain abstract nodes, to be excluded from the whole
path, which include subobjects used to specify the domain
sequence.
o Explicit Exclusion Route Subobject (EXRS): As per [RFC5521], EXRS
can be used to specify exclusion of certain abstract nodes
(including domains) between a specific pair of nodes. EXRS is a
subobject inside the IRO.

o Explicit Route Object (ERO): As per [RFC5440], ERO can be used to
specify a computed path in the network. For example, in the case
of H-PCE [RFC6805], a parent PCE can send the domain sequence as a
result in a path computation reply using ERO.
3.4. Include Route Object (IRO)
As per [RFC5440], IRO can be used to specify that the computed path
needs to traverse a set of specified network elements or abstract
nodes.
3.4.1. Subobjects
Some subobjects are defined in [RFC3209], [RFC3473], [RFC3477], and
[RFC4874], but new subobjects related to domain sequence are needed.
This document extends the support for 4-byte AS numbers and IGP
areas.
Value Description
----- ----------------
5 4-byte AS number
6 OSPF Area ID
7 IS-IS Area ID
Note: Identical subobjects are carried in RSVP-TE messages as defined
in [RFC7898].
3.4.1.1. Autonomous System
[RFC3209] already defines 2-byte AS numbers.
To support 4-byte AS numbers as per [RFC6793], the following
subobject is defined:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AS Number (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L: The L bit is an attribute of the subobject as defined in
[RFC3209], and its usage in the IRO subobject is defined in
[RFC7896].
Type: 5 (indicating a 4-byte AS number).

Length: 8 (total length of the subobject in bytes).
Reserved: Zero at transmission; ignored at receipt.
AS Number: The 4-byte AS number. Note that if 2-byte AS numbers are
in use, the low-order bits (16 through 31) MUST be used, and the
high-order bits (0 through 15) MUST be set to zero.
3.4.1.2. IGP Area
Since the length and format of Area ID is different for OSPF and
IS-IS, the following two subobjects are defined below:
For OSPF, the Area ID is a 32-bit number. The subobject is encoded
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OSPF Area ID (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L: The L bit is an attribute of the subobject as defined in
[RFC3209], and its usage in the IRO subobject is defined in
[RFC7896].
Type: 6 (indicating a 4-byte OSPF Area ID).
Length: 8 (total length of the subobject in bytes).
Reserved: Zero at transmission; ignored at receipt.
OSPF Area ID: The 4-byte OSPF Area ID.

For IS-IS, the Area ID is of variable length; thus, the length of the
subobject is variable. The Area ID is as described in IS-IS by the
ISO standard [ISO10589]. The subobject is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | Area-Len | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// IS-IS Area ID //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L: The L bit is an attribute of the subobject as defined in
[RFC3209], and its usage in the IRO subobject is defined in
[RFC7896].
Type: 7 (indicating the IS-IS Area ID).
Length: Variable. The length MUST be at least 8 and MUST be a
multiple of 4.
Area-Len: Variable (length of the actual (non-padded) IS-IS area
identifier in octets; valid values are from 1 to 13, inclusive).
Reserved: Zero at transmission; ignored at receipt.
IS-IS Area ID: The variable-length IS-IS area identifier. Padded
with trailing zeroes to a 4-byte boundary.
3.4.2. Update in IRO Specification
[RFC5440] describes IRO as an optional object used to specify network
elements to be traversed by the computed path. It further states
that the L bit of such subobject has no meaning within an IRO. It
also does not mention if IRO is an ordered or unordered list of
subobjects.
An update to the IRO specification [RFC7896] makes IRO as an ordered
list and includes support for the L bit.
The use of IRO for the domain sequence assumes the updated
specification is being used for IRO, as per [RFC7896].

3.4.3. IRO for Domain Sequence
The subobject type for IPv4, IPv6, and unnumbered Interface IDs can
be used to specify boundary nodes (ABR/ASBR) and inter-AS links. The
subobject type for the AS Number (2 or 4 bytes) and the IGP area are
used to specify the domain identifiers in the domain sequence.
The IRO can incorporate the new domain subobjects with the existing
subobjects in a sequence of traversal.
Thus, an IRO, comprising subobjects, that represents a domain
sequence defines the domains involved in an inter-domain path
computation, typically involving two or more collaborative PCEs.
A domain sequence can have varying degrees of granularity. It is
possible to have a domain sequence composed of, uniquely, AS
identifiers. It is also possible to list the involved IGP areas for
a given AS.
In any case, the mapping between domains and responsible PCEs is not
defined in this document. It is assumed that a PCE that needs to
obtain a "next PCE" from a domain sequence is able to do so (e.g.,
via administrative configuration or discovery).
3.4.3.1. PCC Procedures
A PCC builds an IRO to encode the domain sequence, so that the
cooperating PCEs could compute an inter-domain shortest constrained
path across the specified sequence of domains.
A PCC may intersperse area and AS subobjects with other subobjects
without change to the previously specified processing of those
subobjects in the IRO.
3.4.3.2. PCE Procedures
If a PCE receives an IRO in a Path Computation Request (PCReq)
message that contains the subobjects defined in this document that it
does not recognize, it will respond according to the rules for a
malformed object as per [RFC5440]. The PCE MAY also include the IRO
in the PCEP Error (PCErr) message as per [RFC5440].
The interpretation of the L bit is as per Section 4.3.3.1 of
[RFC3209] (as per [RFC7896]).

In a Path Computation Reply (PCRep), PCE MAY also supply IRO (with
domain sequence information) with the NO-PATH object indicating that
the set of elements (domains) of the request's IRO prevented the PCEs
from finding a path.
The following processing rules apply for a domain sequence in IRO:
o When a PCE parses an IRO, it interprets each subobject according
to the AS number associated with the preceding subobject. We call
this the "current AS". Certain subobjects modify the current AS,
as follows.
* The current AS is initialized to the AS number of the PCC.
* If the PCE encounters an AS subobject, then it updates the
current AS to this new AS number.
* If the PCE encounters an area subobject, then it assumes that
the area belongs to the current AS.
* If the PCE encounters an IP address that is globally routable,
then it updates the current AS to the AS that owns this IP
address. This document does not define how the PCE learns
which AS owns the IP address.
* If the PCE encounters an IP address that is not globally
routable, then it assumes that it belongs to the current AS.
* If the PCE encounters an unnumbered link, then it assumes that
it belongs to the current AS.
o When a PCE parses an IRO, it interprets each subobject according
to the Area ID associated with the preceding subobject. We call
this the "current area". Certain subobjects modify the current
area, as follows.
* The current area is initialized to the Area ID of the PCC.
* If the current AS is changed, the current area is reset and
needs to be determined again by a current or subsequent
subobject.
* If the PCE encounters an area subobject, then it updates the
current area to this new Area ID.

* If the PCE encounters an IP address that belongs to a different
area, then it updates the current area to the area that has
this IP address. This document does not define how the PCE
learns which area has the IP address.
* If the PCE encounters an unnumbered link that belongs to a
different area, then it updates the current Area to the area
that has this link.
* Otherwise, it assumes that the subobject belongs to the current
area.
o In case the current PCE is not responsible for the path
computation in the current AS or area, then the PCE selects the
"next PCE" in the domain sequence based on the current AS and
area.
Note that it is advised that PCC should use AS and area subobjects
while building the domain sequence in IRO and avoid using other
mechanisms to change the "current AS" and "current area" as described
above.
3.5. Exclude Route Object (XRO)
XRO [RFC5521] is an optional object used to specify exclusion of
certain abstract nodes or resources from the whole path.
3.5.1. Subobjects
Some subobjects are to be used in XRO as defined in [RFC3209],
[RFC3477], [RFC4874], and [RFC5520], but new subobjects related to
domain sequence are needed.
This document extends the support for 4-byte AS numbers and IGP
areas.
Value Description
----- ----------------
5 4-byte AS number
6 OSPF Area ID
7 IS-IS Area ID
Note: Identical subobjects are carried in RSVP-TE messages as defined
in [RFC7898].

3.5.1.1. Autonomous System
The new subobjects to support 4-byte AS numbers and the IGP
(OSPF/IS-IS) area MAY also be used in the XRO to specify exclusion of
certain domains in the path computation procedure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AS Number (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The X-bit indicates whether the exclusion is mandatory or desired.
0: indicates that the AS specified MUST be excluded from the path
computed by the PCE(s).
1: indicates that the AS specified SHOULD be avoided from the inter-
domain path computed by the PCE(s), but it MAY be included subject
to PCE policy and the absence of a viable path that meets the
other constraints.
All other fields are consistent with the definition in Section 3.4.
3.5.1.2. IGP Area
Since the length and format of the Area ID is different for OSPF and
IS-IS, the following two subobjects are defined:
For OSPF, the Area ID is a 32-bit number. The subobject is encoded
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OSPF Area ID (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The X-bit indicates whether the exclusion is mandatory or desired.
0: indicates that the OSPF area specified MUST be excluded from the
path computed by the PCE(s).

1: indicates that the OSPF area specified SHOULD be avoided from the
inter-domain path computed by the PCE(s), but it MAY be included
subject to PCE policy and the absence of a viable path that meets
the other constraints.
All other fields are consistent with the definition in Section 3.4.
For IS-IS, the Area ID is of variable length; thus, the length of the
subobject is variable. The Area ID is as described in IS-IS by the
ISO standard [ISO10589]. The subobject is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Type | Length | Area-Len | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// IS-IS Area ID //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The X-bit indicates whether the exclusion is mandatory or desired.
0: indicates that the IS-IS area specified MUST be excluded from the
path computed by the PCE(s).
1: indicates that the IS-IS area specified SHOULD be avoided from the
inter-domain path computed by the PCE(s), but it MAY be included
subject to PCE policy and the absence of a viable path that meets
the other constraints.
All other fields are consistent with the definition in Section 3.4.
All the processing rules are as per [RFC5521].
Note that if a PCE receives an XRO in a PCReq message that contains
subobjects defined in this document that it does not recognize, it
will respond according to the rules for a malformed object as per
[RFC5440].
IGP area subobjects in the XRO are local to the current AS. In case
multi-AS path computation excludes an IGP area in a different AS, the
IGP area subobject should be part of EXRS in the IRO to specify the
AS in which the IGP area is to be excluded. Further, policy may be
applied to prune/ignore area subobjects in XRO after a "current AS"
change during path computation.

3.6. Explicit Exclusion Route Subobject (EXRS)
The EXRS [RFC5521] is used to specify exclusion of certain abstract
nodes between a specific pair of nodes.
The EXRS can carry any of the subobjects defined for inclusion in the
XRO; thus, the new subobjects to support 4-byte AS numbers and the
IGP (OSPF / IS-IS) area can also be used in the EXRS. The meanings
of the fields of the new XRO subobjects are unchanged when the
subobjects are included in an EXRS, except that the scope of the
exclusion is limited to the single hop between the previous and
subsequent elements in the IRO.
The EXRS should be interpreted in the context of the current AS and
current area of the preceding subobject in the IRO. The EXRS does
not change the current AS or current area. All other processing
rules are as per [RFC5521].
Note that if a PCE that supports the EXRS in an IRO parses an IRO,
and encounters an EXRS that contains subobjects defined in this
document that it does not recognize, it will act according to the
setting of the X-bit in the subobject as per [RFC5521].
3.7. Explicit Route Object (ERO)
ERO [RFC5440] is used to specify a computed path in the network.
PCEP ERO subobject types correspond to RSVP-TE ERO subobject types as
defined in [RFC3209], [RFC3473], [RFC3477], [RFC4873], [RFC4874], and
[RFC5520]. The subobjects related to the domain sequence are further
defined in [RFC7898].
The new subobjects to support 4-byte AS numbers and the IGP
(OSPF/IS-IS) area can also be used in the ERO to specify an abstract
node (a group of nodes whose internal topology is opaque to the
ingress node of the LSP). Using this concept of abstraction, an
explicitly routed LSP can be specified as a sequence of domains.
In case of H-PCE [RFC6805], a parent PCE can be requested to find the
domain sequence. Refer to the example in Section 4.6 of this
document. The ERO in reply from the parent PCE can then be used in
per-domain path computation or BRPC.
If a PCC receives an ERO in a PCRep message that contains a subobject
defined in this document that it does not recognize, it will respond
according to the rules for a malformed object as per [RFC5440].